Add signature with each key for directory responses

[thibmeu]

This adds a method to http-message-sig package so that directory responses can be signed as recommended in the draft. The draft change is pending and available in thibmeu/http-message-signatures-directory#40

This commit also updates the example worker to provide a signature over the directory.

Tests are to be added. They could also serve as a basis to get test vectors in the directory draft.

If needed, this commit can be deployed to a test domain @AkshataDM

When running npm run start locally, I get

$ http http://localhost:8787/.well-known/http-message-signatures-directory
HTTP/1.1 200 OK
Content-Length: 178
Content-Type: application/http-message-signatures-directory
Signature: binding0=:i23cuESh2w4oCNrD+TgsaJXDQf2big85jY3aPV0VTSkZaPQvxqMM6j9r8A3i/D2yzl+lDuDbxrWBlwnUciSkAA==:
Signature-Input: binding0=("@authority");created=1749805161;keyid="poqkLGiymh_W0uP6PZFw-dvez3QJT5SolqXBCW38r0U";alg="ed25519";expires=1749805461;tag="http-message-signatures-directory"

{"keys":[{"kid":"poqkLGiymh_W0uP6PZFw-dvez3QJT5SolqXBCW38r0U","kty":"OKP","crv":"Ed25519","x":"JrQLj5P_89iXES9-vFgrIy29clF9CC_oPPsw3c5D0bs","nbf":1743465600000}],"purpose":"rag"}