Draft: Add TwoTrackKsDownstream and TwoTrackL0Downstream HLT1 physics lines

The MRs adds two new HLT1 downstream physics lines, targeting Ks and L0 decays.

Vertexing and downstream line definition based on !1198 (merged).

Selections are based on TwoTrackKs line ones (long candidates) for Ks decays; in case of L0 decays implemented selections has been taken from a past optimization for L0 decays, but never implemented (more info here https://cds.cern.ch/record/2864032?ln=it).

Selections are not meant to be definitive, just a starting point waiting for further tuning.

Partially addresses #473.

Rate estimate performed on expected 2024 conditions MC are:

Hlt1TwoTrackKsDownstream:                              44/ 18736, (   70.45 +/-    10.61) kHz
Hlt1TwoTrackL0Downstream:                               3/ 18736, (    4.80 +/-     2.77) kHz

Efficiency estimates WIP.