crypto/x509: add additional convenience fields to CertificateRequest

Adds the following additional convenience fields to CertificateRequest:
* KeyUsage
* ExtKeyUsage
* UnknownExtKeyUsage
* IsCA
* MaxPathLen
* BasicConstraintsValid
* MaxPathLenZero
* SubjectKeyId
* PolicyIdentifier

These fields are parsed during ParseCertificateRequest and marshalled
during CreateCertificateRequest. The parsing/marshalling code is
factored out of parseCertificate and buildExtensions (which is renamed
buildCertExtensions). This has the side effect of making these methods
somewhat easier to read.

Documentation for the fields is copied from Certificate.

Example CSR created with all of these fields parsed with openssl:
$ openssl req -in ~/test-csr.pem -noout -text
Certificate Request:
    Data:
        Version: 0 (0x0)
        Subject:
        Subject Public Key Info:
            Public Key Algorithm: id-ecPublicKey
                Public-Key: (256 bit)
                pub:
                    04:a4:cb:64:35:8e:dd:8c:2b:a6:f1:aa:39:d1:be:
                    d0:b9:95:1e:59:19:82:76:28:d3:85:1b:c6:88:62:
                    e1:15:33:be:26:18:80:14:fe:f4:d4:91:66:4e:a4:
                    a4:47:bd:53:db:f7:2e:e3:31:ce:5f:86:cb:92:59:
                    93:bb:d0:7f:a2
                ASN1 OID: prime256v1
                NIST CURVE: P-256
        Attributes:
        Requested Extensions:
            X509v3 Key Usage: critical
                Certificate Sign
            X509v3 Extended Key Usage:
                Any Extended Key Usage, 1.2.3
            X509v3 Basic Constraints: critical
                CA:TRUE, pathlen:0
            X509v3 Subject Key Identifier:
                01:02:03
            X509v3 Certificate Policies:
                Policy: 1.2.3

    Signature Algorithm: ecdsa-with-SHA256
         30:45:02:21:00:a7:88:e5:96:d4:ad:ae:24:26:ab:5f:15:6a:
         3f:22:6d:0e:a6:ba:15:64:8d:78:34:f4:c4:7d:ac:37:b0:2a:
         84:02:20:68:44:f0:8e:8a:1b:c1:68:be:14:a6:e3:83:41:fd:
         2d:cc:00:aa:bc:50:f6:50:56:12:9e:a4:09:84:5c:bf:c1

Fixes #37172

Change-Id: Ife79d01e203827ef0ac3c787aa13c00d0751a1ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/233163


Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Trust: Katie Hockman <katie@golang.org>
Trust: Roland Shoemaker <roland@golang.org>