[SDK-2758] Restore withIssuer

[jimmyjames]

Changes

#288 changed the signature of withIssuer(String issuer) to withIssuer(String... issuer). As discussed in #507, this is a binary incompatible change.

This change restores that binary compatibility, by introducing a new default method on the Verification interface, that simply delegates to the existing implementation.

Notes:

• This change will cause the apiDiff check to fail, as the japicmp gradle plugin does not currently support configuring this check (see Support for overrideCompatibilityChangeParameters melix/japicmp-gradle-plugin#43)
• Rather then configure the plugin to ignore the Verification interface, which would then never catch any issues, we'll need to bump the baseline compare version when this change is released.
• Regarding if adding a default method to an existing interface is binary compatible, this documentation from Oracle provides a good explanation on why it should be considered binary compatible.
• No tests were added as part of this change, as the JWTVerifierTests already contain a test that verify the behavior when calling withIssuer with a single string.
• When this change is released, we will need to update the CHANGELOG for 3.8.0 to indicate the binary incompatible change, and note its fixed version.

[lbalmaceda]

LGTM

[ebickle]

Looks good to me, thank you!