encodebin: Add a way to disable caps renegotiation for output stream format

encodebin: Add a way to disable caps renegotiation for output stream format

In some cases, the user might want the stream outputted by encodebin to
be in the exact same format during all the stream. We should let the
user specify when this is the case. This commit add some API in the
GstEncodingProfile to determine whether the format can be renegotiated
after the encoding started or not.

API:
    gst_encoding_profile_set_allow_dynamic_output
    gst_encoding_profile_get_allow_dynamic_output

https://bugzilla.gnome.org/show_bug.cgi?id=740214