Filters/macros/function for Vector's template syntax

This has been discussed in a variety of issues (...), but we don't have an actual issue open for this work.

Motivation

Our template syntax is very limited in its current form and we've had a number of customers request ways to extend this (#1846, #1692, #1440, https://github.com/timberio/vector/issues/1926#issuecomment-599076830)

Prior Art

1. Liquid filters - Rust implementation
2. Jinja 2 filters

Proposal

We should support "modifiers" or "filters" within our template syntax. For example:

[sinks.archive]
  type = "aws_s3"
  key_prefix = "{{ timestamp | date: "%Y-%m-%d" }}"

Filters

We can use the prior art as a basis, but at the very least we should have:

• default
• strptime
• capitalize
• downcase
• truncate
• upcase

These are most useful for dynamic partitioning.

Start by locating Vector's existing template-syntax entry point and reviewing the Liquid and Jinja filter references linked in the issue. Define the supported filter behavior and integration scope; the work is done when the proposed default, strptime, capitalize, downcase, truncate, and upcase filters are supported.