Remove base64 encoding of participant-context ID in all IdentityHub API URLs

[paullatzelsperger]

As described in Issue #914, the participant context ID in most URL paths of the IdentityHub's IdentityAPI and the IssuerService's AdminAPI must be base64 encoded. This is due to a historic reason where the partiicpantContextID was expected to be a DID, which contains a colon character (":").

Why remove it?

Other APIs of components, that are also participant-context aware, do not have this, as the participantContextId is not expected to be a DID anymore, instead, it should be a randomly generated string such as a UUID.

This will be a breaking change!

[ndr-brt]

also if the APIs are still in alpha version, I would propose to have an intermediary release (0.16) where the participant context id can be either plain or base64, in case the latter, a warning should be logged. This would save a lot of headaches to the adopters.