net: tls_credentials_shell: Add credential buffer load argument

Add argument to the TLS credential `cred buf` command that enables
a shell bypass to write the TLS credential directly to the credential
buffer.
This is useful for writing load credentials that cannot fit in a single
`cred buf` command and would otherwise have to be split into multiple
cred buf commands.
Sending multiple in succession like that from a script for example very
easily causes the shell RX buffer to get full, resulting in multiple
`RX ring buffer full.` warnings.
This is very difficult for a script to handle.

Using a bypass has much better performance and can easily avoid the
RX ring buffer full condition without increasing the RX ring buffer
to much.
It is also easier for a script to use.

Signed-off-by: Joakim Andersson <joerchan@gmail.com>