fix(did-provider-ethr): export KMSEthereumSigner for convenience

[mirceanis]

What issue is this PR fixing

relates to #583

What is being changed

The KMSEthereumSigner class is now exported, but only for convenience.
It is not (yet) meant to be part of the public API.

Quality

Check all that apply:

• I want these changes to be integrated
• I successfully ran pnpm i, pnpm build, pnpm test, pnpm test:browser locally.
• I allow my PR to be updated by the reviewers (to speed up the review process).
• I added unit tests.
• I added integration tests.
• I did not add automated tests because _________, and I am aware that a PR without tests will likely get rejected.

[codecov]

Codecov Report

Base: 80.25% // Head: 84.65% // Increases project coverage by +4.40% 🎉

Coverage data is based on head (2a53232) compared to base (125bf42).
Patch coverage: 81.35% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             next    #1124      +/-   ##
==========================================
+ Coverage   80.25%   84.65%   +4.40%     
==========================================
  Files         118      145      +27     
  Lines        4056    14804   +10748     
  Branches      875     1548     +673     
==========================================
+ Hits         3255    12533    +9278     
- Misses        798     2271    +1473     
+ Partials        3        0       -3     

Impacted Files	Coverage Δ
packages/core-types/src/agent.ts	0.00% <0.00%> (ø)
packages/core-types/src/coreEvents.ts	0.00% <ø> (ø)
packages/core-types/src/index.ts	0.00% <0.00%> (ø)
...s/credential-ld/src/module-types/jsonld/index.d.ts	0.00% <0.00%> (ø)
packages/data-store-json/src/types.ts	0.00% <0.00%> (ø)
...ackages/did-provider-ethr/src/ethr-did-provider.ts	87.09% <ø> (+3.76%)	⬆️
packages/did-provider-ethr/src/index.ts	100.00% <ø> (ø)
packages/did-provider-ethr/src/kms-eth-signer.ts	70.51% <ø> (-7.27%)	⬇️
packages/did-provider-ion/src/functions.ts	95.68% <ø> (ø)
packages/did-provider-ion/src/index.ts	100.00% <ø> (ø)
... and 215 more

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