Add support for SPIFFE cert requests

[jdtw]

The only thing needed to add SPIFFE "support" here is to accept URI-only requests in request-cert. Example workflow:

❯ ./certstrap init --common-name example.com
Created out/example.com.key
Created out/example.com.crt
Created out/example.com.crl

❯ ./certstrap request-cert --uri spiffe://example.com/foo/bar
Created out/spiffe_example.com_foo_bar.key
Created out/spiffe_example.com_foo_bar.csr

❯ ./certstrap sign spiffe_example.com_foo_bar --CA example.com
Created out/spiffe_example.com_foo_bar.crt from out/spiffe_example.com_foo_bar.csr signed by out/example.com.key

❯ certigo dump -v out/spiffe_example.com_foo_bar.crt
** CERTIFICATE 1 **
Input Format: PEM
Serial: 103223067417972581677006756495353345504
Valid: 2021-12-02 20:43 UTC to 2023-06-02 20:52 UTC
Signature: SHA256-RSA
Subject Info:
        CommonName: spiffe://example.com/foo/bar
Issuer Info:
        CommonName: example.com
Subject Key ID: 2E:5A:9A:ED:E7:E6:C8:CA:AB:2C:B0:B1:38:FA:42:5E:28:88:ED:E9
Authority Key ID: 21:10:B3:50:E6:57:10:D2:D5:87:B1:2F:DE:30:EF:F2:14:A7:9F:0C
Key Usage:
        Digital Signature
        Key Encipherment
        Data Encipherment
        Key Agreement
Extended Key Usage:
        Server Auth
        Client Auth
URI Names:
        spiffe://example.com/foo/bar