[proposal] Make headers call for AgentRestClient dynamic

[cre8]

What issue is this PR fixing

Fixes #1313

What is being changed

The AgentRestClient constructor is able to handle functions calls for the headers. This allows to pass dynamic values like an access token that needs to be updated frequently.

Quality

Check all that apply:

• [X] I want these changes to be integrated

• [X] I successfully ran pnpm i, pnpm build, pnpm test, pnpm test:browser locally.

  FAIL  packages/data-store/src/__tests__/data-store-orm.test.ts (25.45 s)
  
  ● Test suite failed to run                                                                                                               
  
  EBUSY: resource busy or locked, unlink './tmp/test-db2.sqlite'                                                                         
  
    143 |   afterAll(async () => {                                                                                                       
    144 |     ;(await dbConnection).close()                                                                                              
  > 145 |     fs.unlinkSync(databaseFile)                                                                                                
        |        ^                                                                                                                       
    146 |   })                                                                                                                           
    147 |                                                                                                                                
    148 |   test('search presentations by verifier', async () => {                                                                       
  
    at Object.<anonymous> (packages/data-store/src/__tests__/data-store-orm.test.ts:145:8)  

  This error has nothing to do with the changes, but resulted in a incomplete test run (OS: Windows 11)

• [X] I allow my PR to be updated by the reviewers (to speed up the review process).

• [ ] I added unit tests.

• [ ] I added integration tests.

• [X] I did not add automated tests because the package has no test implemented, and I am aware that a PR without tests will likely get rejected.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (ac1b620) 85.47% compared to head (3b43ea3) 85.47%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## next #1314 +/- ## ======================================= Coverage 85.47% 85.47% ======================================= Files 170 170 Lines 18950 18952 +2 Branches 2115 2116 +1 ======================================= + Hits 16197 16199 +2 Misses 2753 2753 ```

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