fix: avoid cyclic dependency

[paullatzelsperger]

What this PR changes/adds

This PR moves the factory methods for RetryPolicy and OkHttpClient from the CoreServicesExtension --> DefaultServicesExtension.

Why it does that

This is to avoid cyclic dependencies when attempting to @Inject either of these two into an extension, onto which the CoreServicesExtension in turn has a dependency, e.g. a Vault extension (it surfaced during #1534).

Further notes

The two new @Provider methods were attributed as default providers (isDefault=true) to allow any other extension to override them.
[edit] removed the isDefault again, this caused problems with system tests

Linked Issue(s)

Closes #1775

Checklist

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

[codecov-commenter]

Codecov Report

Merging #1776 (6ed1904) into main (781381c) will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #1776      +/-   ##
==========================================
+ Coverage   67.77%   67.78%   +0.01%     
==========================================
  Files         795      793       -2     
  Lines       16968    16838     -130     
  Branches     1079     1077       -2     
==========================================
- Hits        11500    11414      -86     
+ Misses       5005     4951      -54     
- Partials      463      473      +10     

Impacted Files	Coverage Δ
...dataspaceconnector/core/CoreServicesExtension.java	85.00% <ø> (-2.50%)	⬇️
...nector/core/defaults/DefaultServicesExtension.java	7.69% <0.00%> (-3.42%)	⬇️
...ector/provision/azure/AzureProvisionExtension.java	0.00% <0.00%> (ø)
.../api/multipart/handler/ArtifactRequestHandler.java	61.42% <0.00%> (-4.71%)	⬇️
.../api/multipart/handler/ContractRequestHandler.java	58.33% <0.00%> (-3.49%)	⬇️
...iation/ProviderContractNegotiationManagerImpl.java	90.00% <0.00%> (-0.56%)	⬇️
...ultipart/dispatcher/sender/IdsMultipartSender.java	79.27% <0.00%> (-0.19%)	⬇️
...connector/ids/api/multipart/util/ResponseUtil.java	98.88% <0.00%> (-0.04%)	⬇️
...aceconnector/ids/core/IdsCoreServiceExtension.java	0.00% <0.00%> (ø)
...ctor/ids/spi/domain/iam/DynamicAttributeToken.java	100.00% <0.00%> (ø)
... and 94 more

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