connector/ldap: support the StartTLS flow for secure connections

When connecting to an LDAP server, there are three ways to connect:

1. Insecurely through port 389 (LDAP).
2. Securely through port 696 (LDAPS).
3. Insecurely through port 389 then negotiate TLS (StartTLS).

This PR adds support for the 3rd flow, letting dex connect to the
standard LDAP port then negotiating TLS through the LDAP protocol
itself.

See a writeup here:

http://www.openldap.org/faq/data/cache/185.html

connector/ldap/testdata/ca.crt

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connector/ldap/testdata/ca.key

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connector/ldap/testdata/server.crt

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+-----END CERTIFICATE-----

connector/ldap/testdata/server.key

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+-----END RSA PRIVATE KEY-----