Use originated time from MRT peer table RIB entries

[ties]

The RIB entries in the MRT table dump message contain the originated time. I noticed that some applications process this time, and use it to give a more precise timestamp for when a prefix was originally announced.

rfc6396 section 4.3.4:

[4.3.4](https://datatracker.ietf.org/doc/html/rfc6396#section-4.3.4).  RIB Entries

   The RIB Entries are repeated Entry Count times.  These entries share
   a common format as shown below.  They include a Peer Index from the
   PEER_INDEX_TABLE MRT record, an originated time for the RIB Entry,
   and the BGP path attribute length and attributes.  All AS numbers in
   the AS_PATH attribute MUST be encoded as 4-byte AS numbers.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Peer Index            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Originated Time                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Attribute Length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                    BGP Attributes... (variable)
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 10: RIB Entries

I also validated that a project that uses bgpdump actually uses this orginated time. As a user I would like to use this value when using a parser naïevely on a RIB, e.g.

let parser = BgpkitParser::from_reader(reader);

        for elem in parser {
          # As a user the originated time is more relevant than the time in the MRT header

https://github.com/RIPE-NCC/bgpdump/blob/010381c6a7897a88040f3a1729e7631466e0c00d/bgpdump_lib.c#L663