New HTTP endpoint for creating verifiable credentials

Closes #15

See merge request eclipse/xfsc/tsa/signer!12

Some unit tests start clean and create new signer service instance
for every test case. In order for all test cases to reuse a common
caching document loader, it's injected as dependency to the signer.

All this is needed because recently W3C introduced a rate limiter
on their web site and JSONLD context pages, and unit tests fail if
too many requests for resolving contexts are made in short periods
of time.

The service now supports multiple proofs on VC and VP,
both for creating and verifying them.

integrate helm in project

See merge request eclipse/xfsc/tsa/signer!11

Perform trademark cleanup

See merge request eclipse/xfsc/tsa/signer!10

Check VC proofs before making VP proof

Closes #11

See merge request gaia-x/data-infrastructure-federation-services/tsa/signer!9

Partially resolve "Examples Swagger API needs clarification"

See merge request gaia-x/data-infrastructure-federation-services/tsa/signer!8

Return empty key list when the vault transit engine is empty

Closes #9

See merge request gaia-x/data-infrastructure-federation-services/tsa/signer!7

Authentication middleware is disabled by default

See merge request gaia-x/data-infrastructure-federation-services/tsa/signer!5

Use JWT auth middleware with configuration option

Closes #3

See merge request gaia-x/data-infrastructure-federation-services/tsa/signer!4

New endpoints to retrieve keys namespaces and key names in a given namespace

Closes #7 and #6

See merge request gaia-x/data-infrastructure-federation-services/tsa/signer!3

Additionally this commit changes the URL paths for the endpoints
that retrieve public keys as DID verification methods.

Support separate Vault transit engines as key namespaces

Closes #4 and #2

See merge request gaia-x/data-infrastructure-federation-services/tsa/signer!2

Add Go vulnerability check to CI pipeline

See merge request gaia-x/data-infrastructure-federation-services/tsa/signer!1

Migrate code.vereign.com modules and paths to gitlab.com/gaia-x

See merge request gaiax/tsa-migrate/signer!1

Fix bug with sending additional optional contexts when creating presentation

Closes #23

See merge request !27