Synthesis of p-trifluoroacetamidophenyl 6-deoxy-2-O-3-O- [2-O-methyl-3-O-(2-O-methyl-α-d-rhamnopyranosyl)- α-l-fucopyranosyl]-α-l-rhamnopyranosyl-α-l- talopyranoside: a spacer armed tetrasaccharide glycopeptidolipid antigen of Mycobacterium avium serovar 20