class FileWatcherCertificateProvider

A CertificateProviderInterface implementation that will watch the credential

changes on the file system. This provider will always return the up-to-date

cert data for all the cert names callers set through |TlsCredentialsOptions|.

Several things to note:

1. This API only supports one key-cert file and hence one set of identity

key-cert pair, so SNI(Server Name Indication) is not supported.

2. The private key and identity certificate should always match. This API

guarantees atomic read, and it is the callers' responsibility to do atomic

updates. There are many ways to atomically update the key and certs in the

file system. To name a few:

1) creating a new directory, renaming the old directory to a new name, and

then renaming the new directory to the original name of the old directory.

2) using a symlink for the directory. When need to change, put new

credential data in a new directory, and change symlink.