Make the set of Acquire-By-Hash algorithms configurable

[barnumbirr]

Description of the Change

Acquire-By-Hash entries are written for MD5Sum, SHA1, SHA256 and SHA512,
hardcoded in two places in deb/index_files.go. Every algorithm is a full set of
index copies on each publish, and on S3-like storage each of those is a network
round trip rather than a link.

Measured on an R2-backed archive of 25 packages across three suites, one
aptly publish update per suite, counted at the storage backend:

	S3 requests
Acquire-By-Hash off	38
Acquire-By-Hash on	222 — 88 HeadObject, 83 CopyObject, 35 DeleteObject, 13 PutObject

Modern apt verifies with SHA256; MD5Sum and SHA1 are deprecated and Debian has
been phasing them out of Release files for years. Publishers who do not need
them are paying three quarters of that for nothing, with no way to say so.

acquireByHashAlgorithms in aptly.conf now selects the set:

"acquireByHashAlgorithms": ["SHA256", "SHA512"],

Design notes

• An empty or absent value keeps all four, so existing configurations are
  unaffected and the default behaviour is unchanged.
• Order comes from the code, not the config. The directories are created in
  one loop and the entries written in another; taking the order from the config
  would let those two disagree.
• An unrecognised name is an error, not something to skip. Skipping a typo
  would produce "no by-hash entries for that algorithm", which is
  indistinguishable from having configured it away.
• Set per publish invocation from the config, the way SkipBz2 already is,
  rather than persisted on the publish point.

Independent of #1625

This touches no file that #1625
touches — that one is confined to s3/public.go. Both branch from master and
can merge in either order.

Checklist

• allow Maintainers to edit PR (rebase, run coverage, help with tests, ...)
• unit-test added (if change is algorithm) — deb/index_files_test.go
  covers the default, the configured set, the fixed ordering, and rejection
  of unknown names
• functional test added/updated (if change is functional) — happy to add one
  if you point me at the right fixture
• man page updated — both man/aptly.1.ronn.tmpl and the generated
  man/aptly.1

Test state

This tree has pre-existing failures unrelated to the change; they are unchanged
by it:

package	master	this branch
deb	173 passed, 11 failed	177 passed, 11 failed
utils	20 passed, 1 failed (TestCompress)	20 passed, 1 failed
s3	ok	ok

[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.09%. Comparing base (f59b0d2) to head (9187b90).

Files with missing lines	Patch %	Lines
deb/index_files.go	87.09%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1626      +/-   ##
==========================================
- Coverage   77.37%   77.09%   -0.29%     
==========================================
  Files         165      165              
  Lines       15747    15780      +33     
==========================================
- Hits        12185    12165      -20     
- Misses       2356     2411      +55     
+ Partials     1206     1204       -2     

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