[SPARK-26227][R] from_[csv|json] should accept schema_of_[csv|json] in R API
## What changes were proposed in this pull request?
**1. Document `from_csv(..., schema_of_csv(...))` support:**
```R
csv <- "Amsterdam,2018"
df <- sql(paste0("SELECT '", csv, "' as csv"))
head(select(df, from_csv(df$csv, schema_of_csv(csv))))
```
```
from_csv(csv)
1 Amsterdam, 2018
```
**2. Allow `from_json(..., schema_of_json(...))`**
Before:
```R
df2 <- sql("SELECT named_struct('name', 'Bob') as people")
df2 <- mutate(df2, people_json = to_json(df2$people))
head(select(df2, from_json(df2$people_json, schema_of_json(head(df2)$people_json))))
```
```
Error in (function (classes, fdef, mtable) :
unable to find an inherited method for function ‘from_json’ for signature ‘"Column", "Column"’
```
After:
```R
df2 <- sql("SELECT named_struct('name', 'Bob') as people")
df2 <- mutate(df2, people_json = to_json(df2$people))
head(select(df2, from_json(df2$people_json, schema_of_json(head(df2)$people_json))))
```
```
from_json(people_json)
1 Bob
```
**3. (While I'm here) Allow `structType` as schema for `from_csv` support to match with `from_json`.**
Before:
```R
csv <- "Amsterdam,2018"
df <- sql(paste0("SELECT '", csv, "' as csv"))
head(select(df, from_csv(df$csv, structType("city STRING, year INT"))))
```
```
Error in (function (classes, fdef, mtable) :
unable to find an inherited method for function ‘from_csv’ for signature ‘"Column", "structType"’
```
After:
```R
csv <- "Amsterdam,2018"
df <- sql(paste0("SELECT '", csv, "' as csv"))
head(select(df, from_csv(df$csv, structType("city STRING, year INT"))))
```
```
from_csv(csv)
1 Amsterdam, 2018
```
## How was this patch tested?
Manually tested and unittests were added.
Closes #23184 from HyukjinKwon/SPARK-26227-1.
Authored-by: Hyukjin Kwon <gurwls223@apache.org>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>
39a0493387