Feature: Add action to android-keystore to extract single certificate from a keystore

[priitlatt]

With some keystores keytool is unable to list certificates in RFC format even if -rfc flag is passed to the keytool -list command. For example:

% keytool -list -storepass test123 -keystore /tmp/t.ks.jks -alias debug -rfc
Command line args: [-list, -storepass, test123, -keystore, /tmp/t.ks.jks, -alias, debug, -rfc]
Alias name: debug
Creation date: 12 Aug 2022
Entry type: PrivateKeyEntry
Certificate chain length: 1
Certificate[1]:
[
[
  Version: V3
  Subject: CN=Unknown, OU=Unknown, O=Unknown, L=Unknown, ST=Unknown, C=Unknown
  Signature Algorithm: SHA256withRSA, OID = 1.2.840.113549.1.1.11

  Key:  Sun RSA public key, 2048 bits
...

Due to this we cannot obtain a certificate from a keystore for given alias using the -list verb. However, certificate can be exported in ASN.1 format using -exportcert verb. This PR adds new action android-keystore certificate along with accompanying Python API which is a wrapper for keytool -exportcert.

New action:

• android-keystore certificate