bgpd: fix several issues in sourcing AIGP attribute (backport #17091)

[mergify]

Fix several issues in sourcing AIGP attribute:

1. AIGP should not be set as default for a redistributed route or a
   static network. It should be set by config instead.

2. AIGP sourced by "set aigp-metric igp-metric" in a route-map does
   not set the correct value for a redistributed route.

3. When redistribute a connected route like loopback, the AGIP (with
   value 0) is sourced by "set aigp-metric igp-metric", but the
   attribute is not propagated as the attribute flag is not set.

   ---

   This is an automatic backport of pull request bgpd: fix several issues in sourcing AIGP attribute #17091 done by Mergify.

[mergify]

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Changes to be committed:
	modified:   bgpd/bgp_route.c
	modified:   bgpd/bgp_routemap.c

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   bgpd/bgp_attr.h

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