[JSDK-57] Optional credentials caching + custom pluggable cache

[tl-andrea-dilisio]

Description

One of the feedback from the Security team during their pre go-live review was to make the credentials caching optional. This PR onboards that change and pushes a bit further by making the cache implementation customizable.

To enable the default cache (in memory ) users will have to build a client like this

tlClient = TrueLayerClient.New()
                .clientCredentials(TestUtils.getClientCredentials())
                .signingOptions(TestUtils.getSigningOptions())
                .environment(testEnvironment)
                .withCredentialsCaching()
                .build();

Whereas, if a custom cache is needed, developers can provide an implementation of the ICredentialsCache interface

tlClient = TrueLayerClient.New()
                .clientCredentials(TestUtils.getClientCredentials())
                .signingOptions(TestUtils.getSigningOptions())
                .environment(testEnvironment)
                .withCredentialsCaching(new CustomCache())
                .build();

More on this Wiki page

Type of change

Please select multiple options if required.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code where necessary
• I have added tests that prove my fix is effective or that my feature works
• I have updated the relevant documentation

[CptAlessio]

Thanks Andrea, looks good to me