This commit adds `_if_some()` variants for builder field setters.
The variants aim at improving usability when setting optional fields.
E.g. currently, we need to write something like:
```rust
let opt_value = ...;
let mut sb = gst::Structure::builder("test")
.field("mandatory_field", "mandatory");
if let Some(value) = opt_value
sb = sb.field("optional_field", value);
}
let s = sb.build();
```
With `_if_some()`, this can be written like this:
```rust
let opt_value = ...;
let s = gst::Structure::builder("test")
.field("mandatory_field", "mandatory")
.field_if_some("optional_field", opt_value)
.build();
```
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1424>
