Add function to load the key from x509.pem file

We used to convert a pem certificate file to some intermediate plain
text format; and parse that format under recovery mode. This is
uncessary since the x509.pem can be directly parsed with openssl
functions.

Add the function to load the public key from one x509.pem file and
corresponding unit tests. And we will add more cls to extract the pem
files from otacert.zip later.

Bug: 116655889
Test: verify package with 5 supported certficate versions
Change-Id: Ibc6c696c534567f005db75143cc4ef8d4bdea6a0