Para descobrir quais extensões a AWS permite, execute a query:
SHOW rds.extensions;
Atualizado em 05/10/2018
address_standardizer
address_standardizer_data_us
amcheck
bloom
btree_gin
btree_gist
chkpass
citext
cube
dblink
dict_int
dict_xsyn
earthdistance
fuzzystrmatch
hll
hstore
hstore_plperl
intagg
intarray
ip4r
isn
log_fdw
ltree
orafce
pgaudit
pgcrypto
pgrouting
pgrowlocks
pgstattuple
pg_buffercache
pg_freespacemap
pg_hint_plan
pg_prewarm
pg_repack
pg_stat_statements
pg_trgm
pg_visibility
plcoffee
plls
plperl
plpgsql
pltcl
plv8
postgis
postgis_tiger_geocoder
postgis_topology
postgres_fdw
prefix
sslinfo
tablefunc
test_parser
tsearch2
tsm_system_rows
tsm_system_time
unaccent
uuid-ossp
Instalando PLV8 na AWS
Usando Javascript em funções do PostgreSQL PLV8
Referências
Documentação das extensões disponíveis do PostgreSQL na AWS
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/CHAP_PostgreSQL.html