feat: local Decimal supporting bigint

[Zetazzz]

2609ef8

solution:

create a separate helper file "decimal.ts" to export Decimal from cosmjs or the helper file.

options to add:

prototypes.typingsFormat.customTypes.useAgoricDec | default to undefined

[Zetazzz]

Based on the feedback from agoric team, we need to change the flag name from:

prototypes.typingsFormat.customTypes.useAgoricDec(default to undefined)

to:

prototypes.typingsFormat.customTypes.useCosmjsDecimal(default to true)

And all related logic need to be changed.