BGP Prefix-SID unknown TLV support is broken

[slankdev]

https://tools.ietf.org/html/draft-ietf-idr-bgp-prefix-sid-27#section-3

For future extensibility, unknown TLVs MUST be ignored and propagated unmodified.

In the above draft defining the specification of Prefix-SID, draft says that unknown TLVs MUST be ignore and propagated. but current implementation doesn't achieve it.

frr/bgpd/bgp_attr.c

Lines 2276 to 2277 in f36c5df

	else if (type == BGP_PREFIX_SID_SRV6_L3_SERVICE
	|| type == BGP_PREFIX_SID_SRV6_L2_SERVICE) {

I sent the PR around that but that didn't fix the Prefix-SID problem completely, Current implementation is only ignoring partially, it doesn't propagate unknown TLV to another peer. My previous PR is here. #5278

And essential of this topi is no-test status cause this problem. Following task make bgpd's Prefix-SID implementation robust and better. I'll work for that. wait soon.

• bgpd: suppor unknwon TLV of Prefix-SID
• topotestt: add test for unknwon TLV of Prefix-SID