fix: export aggregation bases

[kahoowkh]

Hi @sudo-suhas,

I got the following error when I try to import MetricsAggregationBase and create a new class CustomAggregation extending it.

class CustomAggregation extends elastic_builder_1.MetricsAggregationBase {
                                                       ^

TypeError: Class extends value undefined is not a constructor or null

I noticed the type definitions of the aggregation bases are exported in index.d.ts but the actual classes are not exported in index.js. I exported them in this PR, so the above can be fixed.

Could you please have a look and let me know what you think?