bgpd: Add `set as-path replace <any|ASN>` cmd for route-maps

```
route-map tstas permit 10
 set as-path replace 1
exit
```

Before:

```
donatas-laptop(config-router-af)# do show ip bgp 10.10.10.10/32
BGP routing table entry for 10.10.10.10/32, version 13
Paths: (1 available, best #1, table default)
  Advertised to non peer-group peers:
  192.168.10.65
  65000 1 2 3 123
    192.168.10.65 from 192.168.10.65 (10.10.10.11)
      Origin IGP, metric 0, valid, external, best (First path received)
      Last update: Mon Apr 25 10:39:50 2022
```

After:

```
donatas-laptop(config-router-af)# do show ip bgp 10.10.10.10/32
BGP routing table entry for 10.10.10.10/32, version 15
Paths: (1 available, best #1, table default)
  Advertised to non peer-group peers:
  192.168.10.65
  65000 65010 2 3 123
    192.168.10.65 from 192.168.10.65 (10.10.10.11)
      Origin IGP, metric 0, valid, external, best (First path received)
      Last update: Mon Apr 25 10:40:16 2022
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/doc/user/bgp.rst b/doc/user/bgp.rst
index e7adad91b..6f99b4114 100644
--- a/doc/user/bgp.rst
+++ b/doc/user/bgp.rst
@@ -1983,6 +1983,11 @@ Using AS Path in Route Map
    Prepend the existing last AS number (the leftmost ASN) to the AS_PATH.
    The no form of this command removes this set operation from the route-map.
 
+.. clicmd:: set as-path replace <any|ASN>
+
+   Replace a specific AS number to local AS number. ``any`` replaces each
+   AS number in the AS-PATH with the local AS number.
+
 .. _bgp-communities-attribute:
 
 Communities Attribute