(MINOR) Move crate-level functions for creating signers to new public create_signer mod

[scouten-adobe]

Changes in this pull request

Turns out it is legal in Rust to publicly export a function as a top-level entity in a crate, but such a function does not appear in the documentation for the crate.

For that reason, we're moving the get_signer and get_signer_from_files functions into a new create_signer module.

(Update: This turned out to be an incorrect assumption on my part, but we've decided to make this change anyway because the docs are easier to comprehend in the new format.)

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented.
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment.

[codecov-commenter]

Codecov Report

Merging #72 (b905488) into main (df9f915) will decrease coverage by 0.01%.
The diff coverage is 26.78%.

@@            Coverage Diff             @@
##             main      #72      +/-   ##
==========================================
- Coverage   73.43%   73.42%   -0.02%     
==========================================
  Files          64       64              
  Lines       13279    13279              
==========================================
- Hits         9752     9750       -2     
- Misses       3527     3529       +2     

Impacted Files	Coverage Δ
sdk/src/lib.rs	100.00% <ø> (ø)
sdk/src/manifest.rs	88.20% <ø> (ø)
sdk/src/openssl/mod.rs	77.77% <ø> (ø)
sdk/src/utils/test.rs	89.78% <0.00%> (ø)
sdk/src/create_signer.rs	25.92% <25.92%> (ø)
make_test_images/src/make_test_images.rs	73.36% <100.00%> (ø)
sdk/src/openssl/ec_signer.rs	84.86% <0.00%> (-1.32%)	⬇️

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[gpeacock]

We are currently not documenting anything behind a feature, because that is the default used when publishing via cargo docs. To fix that we need to add this to the c2pa.rs Cargo.toml file:

[package.metadata.docs.rs]
all-features = true

If we make this change, do we still need this PR?

[scouten-adobe]

The missing-doc issue is mostly addressed via #73, but I would like to discuss this PR as a possibly-preferred API pattern. (A preliminary local test suggests rustdoc doesn't handle this case well, even with the new flags introduced in #73.)

[gpeacock]

I'm ok with this. I would be cleaner if Signer was a struct and these were constructor methods on it, but since Signer is a trait, this makes sense.

[scouten-adobe]

@gpeacock agreed that this would be cleaner if we could make these static methods on Signer itself, but I've re-checked and did not find a way to do that. Merging as is.