API for specifying customized transforms in ChiselStage (and dependencies?)

**Type of issue**: Feature Request

**Is your feature request related to a problem? Please describe.**

We have some (lightweight) customized transforms before emitting the FIRRTL. They are still written in Scala. Currently we have to [build our own stage](https://github.com/OpenXiangShan/XiangShan/blob/master/src/chisel/main/scala/top/XiangShanStage.scala#L49-L68) with the transform in ChiselStage preserved and customized transform inserted.

Is there any clear API for the users to inject some transforms into the stage? The difficult thing may be specifiying the dependency.

**Describe the solution you'd like**

Allowing customized transforms only just after `Convert`? Then the dependency issue does not exist.

**Describe alternatives you've considered**

We are copying the code of ChiselStage (the transforms) and build our own Stage.

**Additional context**
Include detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. Stack Overflow, gitter, [Scastie](https://scastie.scala-lang.org/KtzZQ3nFTea9KoNh0tRqtg).

**What is the use case for implementing this feature?**

Customized transforms to collect information before emitting FIRRTL.

Start by reading the linked XiangShanStage.scala example and the existing ChiselStage implementation, focusing on how transforms are preserved and where Convert runs. Determine the intended injection point and how transform dependencies should be specified. Done means a documented, supported API that lets users add customized transforms without copying ChiselStage.

Written by the indexing model from the issue text.