FsVault: Add Parsers implementation in FsPrivateKeyResolver to support PrivateKeyResolver for PrivateKeyWrapper

[cpeeyush]

What this PR changes/adds

• Moved common PrivateKeyResolver parser code into AbstractPrivateKeyResolver class.
• Adds support for parser in FsPrivateKeyResolver. (Now it is able to parse keys to PrivateKeyWrapper).
• Updated FsPrivateKeyResolver implemenation to store PEM encode string rather than PrivateKey object to make it more generic.
• Adds a new EC key to test keystore. To cover EC keys as well during parser & tests.
• Test coverage.

Why it does that

This change helps in resolving private keys in different formats when using File System Vault by providing appropriate parsers. For instance, in MVD when we use FileSystemVault for testing purpose there IdentityDidCoreExtension generates an exception as FsPrivateKeyResolver.resolvePrivateKey is not able to parse a ECKey from keystore to PrivateKeyWrapper.class

Linked Issue(s)

Relates to https://github.com/agera-edc/MinimumViableDataspace/issues/56

Checklist

• [x] added appropriate tests?
• [x] performed checkstyle check locally?
• [x] added/updated copyright headers?
• [ ] documented public classes/methods?
• [ ] added/updated relevant documentation?
• [x] added relevant details to the changelog? (skip with label no-changelog)
• [ ] formatted title correctly? (take a look at the CONTRIBUTING and styleguide for details)

[codecov-commenter]

Codecov Report

Merging #322 (bb026ee) into feature/1658/add-parsers-fs-vault (2223347) will decrease coverage by 0.01%. The diff coverage is 76.47%.

@@                          Coverage Diff                          @@
##           feature/1658/add-parsers-fs-vault     #322      +/-   ##
=====================================================================
- Coverage                              67.31%   67.30%   -0.02%     
=====================================================================
  Files                                    771      772       +1     
  Lines                                  16599    16615      +16     
  Branches                                1050     1051       +1     
=====================================================================
+ Hits                                   11173    11182       +9     
- Misses                                  4945     4951       +6     
- Partials                                 481      482       +1     

Impacted Files	Coverage Δ
...onnector/spi/security/VaultPrivateKeyResolver.java	66.66% <0.00%> (-15.16%)	:arrow_down:
...nnector/core/security/fs/FsPrivateKeyResolver.java	66.66% <76.47%> (-3.93%)	:arrow_down:
...ector/spi/security/AbstractPrivateKeyResolver.java	81.25% <81.25%> (ø)

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[github-actions[bot]]

Unit Test Results

     429 files  ±0       429 suites  ±0   23m 58s :stopwatch: +35s 12 469 tests +1  12 455 :heavy_check_mark: +1  14 :zzz: ±0  0 :x: ±0  12 536 runs  +1  12 522 :heavy_check_mark: +1  14 :zzz: ±0  0 :x: ±0 

Results for commit bb026ee6. ± Comparison against base commit 22233470.

:recycle: This comment has been updated with latest results.