add extension to js_serializer

[oxarbitrage]

this pull is a replacement for #966 for issue #947

here is how it looks now. for call_order_update:

call_order_update_operation_options = new Serializer(
    "call_order_update_operation_options"
    target_collateral_ratio: optional uint16
)

call_order_update = new Serializer(
    "call_order_update"
    fee: asset
    funding_account: protocol_id_type "account"
    delta_collateral: asset
    delta_debt: asset
    extensions: call_order_update_operation_extensions
)

for account_create:

account_create_operation_ext = new Serializer(
    "account_create_operation_ext"
    null_ext: optional void
    owner_special_authority: optional static_variant [
    no_special_authority
    top_holders_special_authority
]
    active_special_authority: optional static_variant [
    no_special_authority
    top_holders_special_authority
]
    buyback_options: optional buyback_account_options
)

account_create = new Serializer(
    "account_create"
    fee: asset
    registrar: protocol_id_type "account"
    referrer: protocol_id_type "account"
    referrer_percent: uint16
    name: string
    owner: authority
    active: authority
    options: account_options
    extensions: account_create_operation_extensions
)

and for account_update:

account_update_operation_ext = new Serializer(
    "account_update_operation_ext"
    null_ext: optional void
    owner_special_authority: optional static_variant [
    no_special_authority
    top_holders_special_authority
]
    active_special_authority: optional static_variant [
    no_special_authority
    top_holders_special_authority
]
)

account_update = new Serializer(
    "account_update"
    fee: asset
    account: protocol_id_type "account"
    owner: optional authority
    active: optional authority
    new_options: optional account_options
    extensions: account_update_operation_extensions
)

Problem i see is that the new serializers don't have the exact same name as the extension field(account_update_operation_extensions is serialized as account_update_operation_ext.

As i dont know exactly how this is used by the UI i am unsure if it is an issue or not.

[cogutvalera]

Looks good to me. Seems all is OK here.
All tests have passed with No errors detected. Nice job ! Well done ! Thanks !

PS: just require rebase.

[pmconrad]

Closed #947 in favour of #1701 - perhaps this PR can be integrated into a broader solution.