refactor(storage/sql): embed migrations into flipt binary

[GeorgeMac]

The core of this refactor is to move migrations into an embed.FS variable.
This will embed all migrations into the Flipt binary; making it more portable.

The PR:

1. Creates the new package config/migrations containing a single exported variable FS with go:embed directive.
2. Updates all the locations where we source migrations via file, to use the new source driver iofs.
3. Deprecates both db.migrations.path and db.migrations_path configuration options.
4. Removes all references internally to these deprecated paths.
5. Removes references to config/migrations from packaging, archiving and docker image building.
6. Does a little shuffling around of storage/sql to introduce a new re-usable storage/sql/testing package.

[markphelps]

Looking good, mostly just minor pedantic comments. Gonna pull down locally real quick to get a better look before 👍🏻 .

Also would you mind adding a section to the depcrecations file ? Also perhaps adding a section to the Changelog like:

## Unreleased

### Deprecated
- blah

Just so we don't forget before releasing?

In the future we could prob move to using release-please to update the changelog for us based on conventional commits, but we may need to do some configuration there first to make it match the current syntax of our Changelog file.

[markphelps]

This is great btw, will prevent issues in the future with migrations misconfigurations/incorrect paths! TY!!

[codecov-commenter]

Codecov Report

Merging #1096 (a7b276b) into main (56e3241) will decrease coverage by 0.28%.
The diff coverage is 84.52%.

@@            Coverage Diff             @@
##             main    #1096      +/-   ##
==========================================
- Coverage   80.80%   80.52%   -0.29%     
==========================================
  Files          26       26              
  Lines        1870     1884      +14     
==========================================
+ Hits         1511     1517       +6     
- Misses        279      287       +8     
  Partials       80       80              

Impacted Files	Coverage Δ
internal/storage/sql/migrator.go	26.38% <0.00%> (-0.76%)	⬇️
internal/storage/sql/db.go	90.69% <68.42%> (-4.09%)	⬇️
internal/config/database.go	100.00% <100.00%> (ø)
internal/storage/sql/metrics.go	60.90% <100.00%> (+0.72%)	⬆️

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[markphelps]

👨🏻‍🍳 💋